This invention relates to a method and a system for a relative calibration of the signal path in a radio base station from the point of carrier modulation to the antenna connector and more specifically applies to array antennas used for continuous mobile tracking and for nulling out interference.
Today""s implementation of calibration is to completely avoid it, or to calibrate only within the radio base station itself. In order to avoid calibration of transmitting paths the usual implementation is to use fixed beam solutions instead. Thereby, only one of the feed cables has to be used for a specific carrier and beam direction. No calibration of neither cables nor any other transmitter part (TX) will be necessary. Beam-forming is then made by preferably placing a Butler matrix nearby the antenna radiators at the top of the mast giving the necessary phase distribution to the elements. Only one port has to be fed to give excitation to all antenna elements using a Butler matrix or anther similar beam-forming network.
The drawback of today""s solution is that the feed cables are usually not included in the calibration, but only the components within the base station, if calibration is implemented at all. Fixed beam implementations of array antennas are instead used. This in turn means that nulling out interfering signals from mobiles is not possible, and gain loss is at hand for mobiles located between two beams. Gain loss means that two beams intersect each other at some point (usually xe2x88x923 dB), which decreases the power that can be transmitted/received. Moreover, mobiles cannot be continuously tracked when they move. Instead the one of the beams which gives the best signal strength is chosen. Also, the overall drawback is less traffic capacity possible and/or speech quality.
Calibration all the way up to the antenna connectors might be implemented by inserting a calibration signal at the appropriate frequency somewhere along the signal path. Or alternatively, the traffic signal itself might be used for the calibration. Then signal would be fed back again down to the base station by either extra RF cables (as many as antenna feed cables) or by shifting the frequency to receive frequency band and reading the signals using the ordinary receivers in the base stations.
An international application WO97/44920 discloses an array antenna, which performs self-calibration by providing weighting factors for the adaptive array of antenna elements by modulating a narrow band channel with a relatively broadband signal containing a test sequence. In this way an overlaid channel is produced which is picked up and carried back through a complimentary one of the reception or transmission paths to thereafter be converted to base-band and compared with the test sequence for detecting and correcting possible errors.
The problem with the aforementioned method is that it occupies at least one channel in receive or transmit path and possibly also requires additional RF cables up-and-down the antenna mast just for calibration. Moreover, the transmission down to the base station also needs calibration for the RF frequency at use. This all makes it a very complicated method to calibrate antenna arrays using this method.
A need therefore exists for an improved array antenna calibration method and system which in an effective and simple way will be able to provide an optimal operation of a base station array antenna.
The present disclosure describes a simple way to perform phase calibration, which needs only a limited set of additional hardware, and can be used under on-going traffic conditions. The proposed method in this disclosure makes it possible to implement calibration without substantial additional complexity to the base station system. Moreover, also antenna feed cables may be calibrated. For the calibration additional RF cables are not necessary.
A method and a system for an array antenna calibration are disclosed. The method and system provide a sensor system, localized close to the antenna elements, presenting a sensor for each antenna element, whereby the sensors constitute simple digital receivers and produce complex base-band signals. The outputs of the sensor system digital receivers are interconnected for vector-adding the outputted base-band signals to produce a summed DC voltage. The acquired summed DC voltage is via a device, for instance a Digital Signal Processing cluster, controlled by adjusting the phases of each antenna element source signal and by searching for a maximum summed DC voltage from the sensor system, whereby the individual signal paths of the array antenna will be calibrated. Normally when a calibration is performed, the antenna beam would need to always steer in the broadside direction. To facilitate calibration during normal operation regardless of intended phase settings, intended offset phases are accounted for in the calibration process. These offset phases provided for changing the beam direction of the adaptive array antenna are supplied to the summing step to incorporate those offsets into the calibration algorithm.
A method for calibration of an array antenna according to the present invention is set forth by the independent claim 1 and the dependent claims 2-7 and a system for calibration of an array antenna is set forth by the independent claim 8 and further embodiments of the system are set forth by the dependent claims 9-14.